In modern vehicles, automatically shiftable transmissions are utilized in vehicle drivelines to enhance vehicle operator comfort and convenience. Typically, up-shifts through forward ratios of such a transmission are accomplished in a power-on mode, i.e., while the engine is supplying input torque to the transmission. Such a transmission may be utilized as part of a conventional powertrain, i.e., employing a single engine, or in a hybrid powertrain, where two or more distinct power sources, such as an engine and an electric motor, are employed to propel the vehicle.
In order to maximize fuel efficiency in a hybrid vehicle, the engine may be shut off when the vehicle is in a coast down mode, i.e., when the vehicle is decelerating from elevated speeds due to road interface and vehicle driveline friction, as well as due to air resistance, or during braking. While the hybrid vehicle is in coast down mode, vehicle inertia may be employed to back-drive the electric motor in generator mode for recharging vehicle batteries, thereby improving efficiency further. Hybrid vehicle deceleration may also be provided via a regenerative braking system, where the engine may likewise be shut off, and the otherwise lost braking energy is similarly recaptured via the electrical motor. When a hybrid vehicle is in a deceleration mode, because its engine is typically shut off, the engine cannot provide torque input to the vehicle's automatic transmission. Consequently, while the hybrid transmission is executing shifts, such an input torque interruption may generate unwanted driveline disturbances.